KR20170035639A - Electrolyte solution for lithium air battery and lithium air battery comprising the same - Google Patents

Abstract

The present invention relates to an electrolyte for a lithium air battery and a lithium air battery including the same and, more specifically, relates to an electrolyte for an air lithium air battery and a lithium air battery including the same, including: composite lithium salt composed of lithium hexafluorophosphate (LiPF_6) and lithium bisfluorosulfonylimide (LiFSI); and a DMSO solvent. The electrolyte for a lithium air battery increases energy efficiency of the lithium air battery by decreasing discharging and charging overload of the battery.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an electrolyte for a lithium air battery and a lithium air battery including the lithium battery,

The present invention relates to a lithium ion battery having improved energy efficiency and an electrolyte solution used therefor.

In order to overcome the environmental problem of the primary battery which can not be recharged, the rechargeable secondary battery has been developed. The rechargeable battery may be a lithium secondary battery, a nickel battery, or a nickel cadmium battery depending on an internal filling material. However, the thickness of the battery is thin and light, and a lithium secondary battery suitable for a high capacity battery occupies most of the secondary battery market.

Lithium secondary batteries are widely used in small- and medium-sized electronic products, and those with high energy consumption, such as electric vehicles and hybrid vehicles, are not suitable due to limitations in battery capacity. That is, an electric vehicle equipped with a lithium secondary battery is difficult to travel over a long distance and requires frequent charging.

In order to solve such a problem, studies on a lithium air battery having a higher energy density than that of a lithium secondary battery have been actively conducted. Lithium air cells have an energy density of 700 to 3,000 mAh / g, which is about 6 to 20 times higher than the energy density of conventional lithium secondary batteries of 120 to 150 mAh / g. The structure of the lithium air battery is composed of a cathode, a separator, a cathode, and an electrolytic solution similarly to a lithium secondary battery, except that the anode uses oxygen in the air as an active material and is advantageous over a lithium secondary battery in terms of raw material supply and demand. Here, the active material refers to a substance that releases or absorbs electrons in a chemical reaction with an electrolytic solution.

On the other hand, in the lithium air battery, lithium ions and electrons are generated by the oxidation / reduction reaction of the lithium metal in the negative electrode during the discharge reaction, the lithium ions move through the electrolyte, and the electrons move to the positive electrode . When the oxygen contained in the outside air flows into the anode, oxygen is reduced by the electrons moved to the anode along the conductor to form Li ₂ O ₂ (see the following reaction formula 1). At this time, the discharge voltage is generally 2.6 to 3.0 V.

[Reaction Scheme 1]

O ₂ + 2Li ⁺ + 2e ^- ↔ (Li ₂ O ₂ ) _solid

The charging reaction proceeds in a reaction opposite to that of Reaction Scheme 1. The charging voltage is about 4 V according to the experimental results. Lithium air cells have a higher charging voltage than discharge voltage. This is because Li ₂ O _{2, which} is a discharge product in the above Reaction Scheme 1, must be decomposed upon charging, since the decomposition reaction of Li ₂ O ₂ is slow.

In order to solve such a problem, Korean Patent Laid-Open No. 10-2013-0099706 discloses a method for improving the energy efficiency of a lithium air battery by using an electrolyte solution containing an ammonium cation, an imidazolium cation, etc. and a lithium salt, lithium ion and a conductive polymer Respectively. However, Korean Patent Laid-Open Publication No. 10-2013-0099706 improves the ionic conductivity by the electrolytic solution to some extent, but does not promote the decomposition reaction of the discharge product Li ₂ O _2. Therefore, the charge / Is not efficient.

Korean Patent Publication No. 10-2013-0099706, "Electrolyte Solution and Lithium Air Battery Including the Same"

As a result of various studies to solve the above problem, the present inventors have developed an electrolyte solution in which two types of lithium salts are combined with a solvent. As a result of applying this electrolyte as an electrolyte of a lithium air battery, The discharge overvoltage can be significantly lowered and the energy efficiency can be remarkably improved. Thus, the present invention has been completed.

Accordingly, it is an object of the present invention to provide an electrolyte solution for a lithium air battery which can significantly reduce the charging and discharging over-voltage of the battery.

Another object of the present invention is to provide a lithium air battery with improved energy efficiency.

In order to achieve the above object,

Dimethyl sulfoxide (DMSO) solvent; And

There is provided an electrolyte for a lithium air battery comprising a composite lithium salt in which lithium hexafluorophosphate (LiPF ₆ ) and lithium bis (fluorosulfonyl) imide, LiFSI are mixed.

Wherein the composite lithium salt is mixed with lithium hexafluorophosphate and lithium bisfluorosulfonylimide in a weight ratio of 1: 2 to 2: 1.

In addition,

An anode, a cathode, a separator interposed therebetween, and an electrolyte,

And a lithium air battery including the electrolyte solution for a lithium air battery as described above.

The electrolyte for a lithium air battery according to the present invention uses an electrolytic solution in which a mixed lithium salt is added to a dimethylsulfoxide solvent to easily decompose the discharge product Li ₂ O ₂ which is a cause of charging overvoltage and drastically lower the charging overvoltage, Thereby reducing the voltage.

As a result, when this is applied to a lithium air battery, an effect of improving energy efficiency can be secured.

1 is a graph showing charge / discharge curves of a lithium ion battery according to an embodiment of the present invention and a comparative lithium ion battery.

Hereinafter, the present invention will be described in detail.

(1) electrolyte for lithium air battery

The electrolyte solution for a lithium air battery of the present invention is a form in which a composite lithium salt is dissolved in a solution. The electrolytic solution conventionally used includes an aqueous electrolytic solution in which a lithium salt is added to a water solvent and a non-aqueous organic solvent to which a lithium salt is added in an organic solvent. Conventional electrolytes include organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), dibutyl carbonate DBC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), fluoroethylene carbonate (FEC), dibutyl ether, dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran 1,3-dioxolane, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane, acetonitrile, dimethyl But are not limited to, formamide, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, Butyrolactone, 2-methyl-γ-butyrolactone, 3-methyl-γ-butyrolactone, 4-methyl-γ-butyrolactone , tris (2-chloroethyl) phosphate, tris (2,2,2-trifluoroethyl) phosphate, tripropyl phosphate, triisopropyl phosphite, triisopropyl phosphite, triisopropyl phosphite, (Li ₂ O ₂ ) decomposition rate, which is a discharge product, was not influenced by the use of propyl phosphate, tributyl phosphate, trihexyl phosphate, triphenyl phosphate, tritolyl phosphate and polyethylene glycol dimethyl ether Charging overvoltage is generated.

Accordingly, in the present invention, a non-aqueous liquid electrolyte is used as the electrolytic solution, a composite lithium salt is used as a lithium salt, and dimethyl sulfoxide (DMSO) is used as a solvent.

Unlike other non-aqueous solvents, DMSO has been found to be more effective in inducing the oxidation-reduction reaction of formula (1) to a more reversible reaction and decomposing Li ₂ O ₂ .

In the electrolyte of the present invention, a composite lithium salt is added to DMSO, which is a mixture of lithium hexafluorophosphate (hereinafter referred to as LiPF ₆ ) represented by the following formula 1 and lithium bisfluoro Lithium bis (fluorosulfonyl) imide (hereinafter referred to as " LiFSI ").

[Chemical Formula 1]

(2)

Depending on the molarity of the complex lithium salt, it affects the migration rate of lithium ions and the decomposition rate of Li ₂ O _2. LiMn ₂ O ₃ may be added to the DMSO solvent by adding LiMF _{6 in the range} of 0.1M to 0.7M and LiFSI in the range of 0.1M to 0.7M have. When LiPF ₆ or LiFSI is added at a concentration of less than 0.1 M, the decomposition rate of Li ₂ O ₂ , which is a discharge product, can not be improved so that the charging voltage can not be lowered and charging capacity can be lowered. It does not have adequate conductivity and viscosity as an electrolyte solution and can not exhibit the performance of the electrolyte solution. Preferably, 0.5 M lithium hexafluorophosphate and 0.5 M lithium bisfluorosulfonylimide are added to the dimethylsulfoxide solvent to significantly reduce the charging voltage and ensure efficient charging capacity.

On the other hand, as the mixing ratio, LiPF ₆ or LiFSI mixed in a molar concentration range of 1: 2 to 2: 1 may be used, and more preferably mixed at a ratio of 1: 1. If the mixing ratio is out of the range, the decomposition rate of Li ₂ O ₂ is lowered and the charge capacity is lowered.

As described above, the electrolyte solution of the present invention stably induces the reversible reaction (Reaction 1) of the lithium air battery, greatly reducing the charging voltage and decreasing the discharging voltage, thereby improving the energy efficiency.

(2) Lithium air battery

In addition, the present invention can be applied to the electrolyte solution of a lithium air battery. For example, in a lithium air battery, a separator is disposed between an anode (or an air electrode), a cathode facing away from the anode, and an anode and a cathode, and an electrolyte is injected thereinto.

When an electrolyte solution containing a lithium salt of LiPF ₆ and LiFSI in a DMSO solvent is applied to a lithium air cell, the charge over-voltage is significantly lowered and the discharge over-voltage is lowered to improve energy efficiency.

The electrode of the lithium air battery includes an electrode current collector and an electrode coalescing layer on one or both surfaces of the electrode current collector. In this case, the electrode current collector is a positive current collector when the electrode is a positive electrode, and is a negative electrode current collector when the electrode is a negative electrode.

The positive electrode of the lithium air battery uses oxygen in the air as an active material, and the positive electrode may have a porous form so that external oxygen can be smoothly introduced. The material of the anode is not particularly limited as long as it is a conductive metal in which porous pores can be formed, and preferably a carbon structure having porous pores is used as the anode.

The diameter of the pores of the carbon support may be 2 to 1000 nm in consideration of the flow of oxygen as an active material and the size of the catalyst to be supported. Here, the carbon structure includes carbon nanotubes, mesoporous carbon, graphite, graphite, carbon black, acetylene black, denka black, ketjen black, carbon fiber, fullerene and activated carbon. Preferably, the carbon nanotubes having high porosity are used as an anode to improve the decomposition rate of Li ₂ O ₂ , which is a discharge product, during charging, and to effectively lower charge / discharge overvoltage.

In the lithium air battery of the present invention, in order to lower the activation energy required for the oxidation and reduction reaction of oxygen gas, a catalyst may be supported on the porous anode. The catalyst may be selected from the group consisting of Ru, Rh, Pd, Pt, Au, Ag, Ir, Ni, , Osmium, titanium, and alloys thereof. It is preferable to use ruthenium (Ru) which has high activity in both the oxygen reduction reaction at the time of discharge and the oxygen oxidation reaction at the time of discharge and can improve the decomposition rate of Li ₂ O ₂ which is a discharge product during charging. The catalyst may cover the whole or a part of the surface of the porous structure to form a layer, or it may be supported at a spaced distance between the carbon structures.

When the catalyst layer is formed on the surface of the porous structure, the catalyst layer may be formed in all or part of the surface of the porous structure with a thickness of 2 nm to 20 nm. When the catalyst layer is less than 2 nm, it is difficult to uniformly coat the porous structure. When the catalyst layer is larger than 2 nm, a large amount of catalyst is used, which increases the production cost and decreases the energy density per mass. The catalyst layer may be formed on the surface of the porous structure by a method known in the art such as electroless plating, electroplating, virtual deposition, and chemical synthesis.

The size of the particles of the catalyst may be 1 nm or more and 1um or less. In this case, the activity can be effectively shown in both the oxygen reduction reaction and the oxygen oxidation reaction. The content of the catalyst may be 0.1 wt% or more and 20 wt% or less based on the total weight of the anode. If the content of the catalyst is less than 0.1 wt%, the catalytic content is too small to achieve an activity effect on the oxygen reduction reaction and the oxygen oxidation reaction, and if the content is more than 20 wt%, the conductivity decreases and the battery performance may be deteriorated.

The anode may be a binder for increasing the cohesion of the carbon structure and the bonding force between the catalyst and oxygen which is an active material. The binder may be selected from the group consisting of polyvinylidene fluoride, poly (vinyl acetate), polyvinyl alcohol, polyethylene oxide, (Ethyl acrylate), polytetrafluoroethylene, polytetrafluoroethylene, polytetrafluoroethylene, polytetrafluoroethylene, polytetrafluoroethylene, polytetrafluoroethylene, polytetrafluoroethylene, polytetrafluoroethylene, At least one selected from the group consisting of fluoroethylene polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polystyrene, derivatives thereof, blends and copolymers.

The positive electrode may contain 70 to 90% by weight of the carbon structure, and 30 to 10% by weight of the binder. If the binder is less than 10 wt%, the physical properties of the anode may deteriorate and the active material and the conductive material may be separated from the anode. If the amount exceeds 30 wt%, the ratio of the active material and the conductive material is relatively decreased. Accordingly, it is preferable that the anode has a weight ratio of the carbon structure to the binder in the range of 7: 3 to 9: 1.

The positive electrode may further include a current collector to which current collecting is performed. For example, the current collector may be coated with a porous carbon structure, and the porous carbon structure may be supported or coated with a catalyst. The current collector may be any material having electrical conductivity, and may be selected from carbon, stainless steel, nickel, aluminum, iron, copper, and mixtures thereof. It is preferable that the current collector has a porous pore shape so that oxygen as an active material can be easily introduced. The current collector can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The lithium air battery of the present invention may further include a cathode. The negative electrode may discharge lithium ions at the time of discharging and may receive lithium ions at the time of charging. The negative electrode may be selected from a lithium metal, a lithium-silicon composite, a lithium-titanium oxide, a lithium-carbon composite, a lithium-polymer composite, a lithium metal-based alloy, a lithium compound and a lithium intercalation material. The lithium metal based alloy may be an alloy of lithium and a metal selected from the group consisting of sodium, potassium, robium, cesium, franc (FR), beryllium, magnesium, calcium, strontium, barium, .

The negative electrode may include a negative electrode current collector and a negative electrode active material layer coated on the negative electrode current collector. At this time, an active material containing lithium is used as the negative active material, and the active material layer may have a plate shape, a sheet shape, a powder shape, or a granule shape. The negative electrode collector performs the collection of the negative electrode, and any material having electrical conductivity may be used. For example, the negative electrode collector may be selected from the group consisting of copper, carbon, stainless steel, nickel, aluminum, iron and titanium. The use of a carbon-coated anode current collector is more effective than a carbon-free anode current collector because of its excellent adhesion to active materials and low contact resistance. The shape of the negative electrode collector may be various forms such as a film, a sheet, a foil, a net porous body, a foam or a nonwoven fabric.

In the lithium ion battery of the present invention, a separator may be disposed between the anode and the cathode. The separator separates or insulates the positive electrode and the negative electrode from each other and enables lithium ion transport between the positive electrode and the negative electrode. Any separator can be used as long as it can pass only lithium ions and block the remainder.

For example, the separator may be made of a porous, nonconductive or insulating material. More specifically, a nonwoven fabric of polypropylene material or a nonwoven fabric of nonwoven fabric of polyphenylene sulfide material, and a porous film of olefin resin such as polyethylene or polypropylene can be exemplified, or two or more of them can be used in combination. Such a separation membrane may be an independent member such as a film, or may be a coating layer added to the anode and / or the cathode. The separation membrane impregnates an electrolyte solution and may be used as a support for the electrolyte solution.

The shape of the lithium air battery is not particularly limited and may be, for example, a cylindrical shape, a coin shape, a flat plate shape, a horn shape, a button shape, a sheet shape or a laminate shape.

The lithium air battery of the present invention can be constituted by a plurality of battery modules assembled into a single unit cell. For example, a bipolar plate may be inserted between the lithium air cells of the present invention to be stacked. The bipolar plate may be porous to supply air supplied from the outside to the positive electrode included in each of the lithium air cells. For example, porous stainless steel or porous ceramics.

The battery module may be specifically used as a power source for an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or a power storage device.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the present invention. Such variations and modifications are intended to be within the scope of the appended claims.

≪ Preparation of Example &

Carbon nanotubes for polymer composites were prepared by using a high - speed fluidized - bed reactor. 0.5 g of the carbon nanotubes was added to distilled water together with 0.1 g of ruthenium trichloride (Rucl ₃ ), followed by ultrasonic wave mixing. Subsequently, the mixture was placed in an autoclave, heated to 180 ° C, held for 12 hours, naturally dried, washed with filtration and distilled water, and vacuum-dried for 12 hours to obtain ruthenium-loaded carbon nanotubes.

Ruthenium (Ru) -supported carbon nanotube and polyvinylidene fluoride (PVDF) binder were mixed at a weight ratio of 8: 2 to prepare a slurry. The slurry was coated on a porous carbon paper (sgl31ba, SGL, USA) and vacuum dried at 130 ° C for 12 hours to prepare a positive electrode.

The prepared anode was processed into a circular electrode having a diameter of 19 mm and then used as an air electrode. A lithium salt of 0.5 M LiFSI (lithium bis (fluorosulfonyl) imide] and a lithium salt of LiPF ₆ (Lithium hexafluorophosphate) Dimethyl sulfoxide (DMSO) was used as an electrolytic solution. A circular glass fiber having a diameter of 19 mm and a thickness of 1 mm was used as a separator and a lithium metal having a diameter of 19 mm was used as a cathode.

<Preparation of Comparative Example 1>

The positive electrode was prepared by mixing Ketjen black and PVDF binder in a weight ratio of 8: 2 (Ketjen black 0.8 g, PVDF 0.2 g) to prepare a slurry. The slurry was coated on a carbon paper and vacuum dried at 130 캜 for 12 hours to prepare a positive electrode. Thereafter, the electrode was fabricated into a circular electrode having a diameter of 19 mm and used for assembling the electrode. The electrolytic solution was prepared by diluting 1 M LiTFSI [lithium bis (trifluoromethanesulfonyl) imide] lithium salt with tetraethylene glycol dimethyl ether (TEGDME). In addition, the separation membrane and the cathode were prepared in the same manner as in Example.

&Lt; Preparation of Comparative Example 2 >

The positive electrode was prepared by mixing a ruthenium (Ru) catalyst-supported carbon nanotube and a PVDF binder in the same manner as in Example. Except that 1 M LiTFSI lithium salt as an electrolyte was mixed with tetraethylene glycol dimethyl ether (TEGDME). In addition, the separation membrane and the cathode were prepared in the same manner as in Example.

Comparative raw materials of Comparative Example 1, Comparative Example 2 and Examples are shown in Table 1 below.

division Comparative Example 1 Comparative Example 2 Example anode Carbon black: PVDF
Binder = 8: 2 Ruthenium (Ru) catalyst supported carbon nanotube: PVDF binder = 8: 2 Electrolyte (Lithium salt) LiFSI 1M LiTFSi 0.5M LiFSI (Lithium salt) LiPF ₆ - - 0.5M LiPF ₆ Electrolyte TEGDME DMSO cathode Li metal (same) week)
PVDF: Polyvinylidene fluoride
LiFSI: lithium bis (fluorosulfonyl) imide]
LiPF ₆ : Lithium hexafluorophosphate &lt; RTI ID = 0.0 &gt;
LiTFSI: Lithium bis (trifluoromethanesulfonyl) imide
TEGDME: Triethylene glycol dimethyl ether
DMSO: Dimethyl sulfoxide

<Experimental Example - Charge / discharge voltage measurement>

Discharge tests of the battery were performed using a potentiostat (bio-Logic, VMP3). The electrochemical experiment of the lithium air battery cell prepared in Examples and Comparative Examples 1 and 2 was conducted by discharging by a voltage cut-off method at a current density of 100 mA / g with respect to the electrode weight and setting the lower limit voltage to 2.8 V Respectively. The prepared cell was filled with pure oxygen at 1.5 atm in a closed space, and the measurement was started after sufficiently wetting the electrolyte for 5 hours so as to sufficiently penetrate the electrode and the separator.

The measured charging voltage and discharging voltage were as shown in Fig. According to the graph shown in FIG. 1, it can be seen that the charging voltage in the embodiment (blue) is greatly reduced compared to Comparative Example 1 (black) and Comparative Example 2 (red).

The charging and discharging voltage ideally generated in the oxidation and reduction reaction of the lithium air battery (see Reaction 1) is about 2.96V. The ideal charging and discharging voltage is caused by the material of the anode, the electrolytic solution, the cathode, etc. constituting the lithium air battery, and the composition and arrangement characteristics (resistance element) of the lithium air battery. This is a model of a lithium-ion battery having an energy efficiency higher than the ideal charging and discharging voltage (2.96 V) by comparing charging and discharging voltages of the examples and comparative examples, and if the difference is small (close to ideal charging and discharging voltage).

Comparative Example 2 is different from Comparative Example 1 in that a carbon nanotube having a ruthenium (Ru) catalyst supported thereon was used as the anode, and that the charging voltage of Comparative Example 2 was lower than that of Comparative Example 1. Comparing the example with the comparative example 2, the embodiment is different from the comparative example 2 in that the electrolyte is a composite lithium salt of 0.5M LiFSi and 0.5M LiPF ₆ in a DMSO solvent, Which is significantly lower than the charging voltage. Therefore, it can be confirmed that the lithium air battery of the embodiment is close to the ideal charging voltage, and the energy efficiency is high.

In the case of the discharge voltage, Comparative Example 1 and Comparative Example 2 were confirmed to be 2.65V. In Comparative Example 1 and Comparative Example 2, an ideal discharge voltage of 2.96 V and a difference of 0.3 V were observed, and an overvoltage of 0.3 V was applied. On the other hand, the discharge voltage of the embodiment was confirmed to be about 2.8V. The embodiment exhibits an ideal discharge voltage difference of 2.96 V and 0.15 V, and an overvoltage of 0.15 V is applied. Therefore, it can be seen that the embodiment is close to an ideal discharge voltage and is more energy efficient than the comparative example.

Therefore, it can be seen that the electrolyte for the lithium air battery of the present invention and the lithium air battery including the same have a high energy efficiency by lowering the charging voltage and the discharging voltage.

While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It is to be understood that the invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. It is therefore to be understood that the above-described embodiments are illustrative and non-restrictive in every respect.

Claims (6)

Dimethyl sulfoxide solvent; And
An electrolyte for a lithium air battery comprising a composite lithium salt in which lithium hexafluorophosphate (LiPF ₆ ) and lithium bisfluorosulfonylimide (LiFSI) are mixed.
The method according to claim 1,
Wherein the composite lithium salt is mixed in a molar ratio of lithium hexafluorophosphate to lithium bisfluorosulfonylimide of 1: 2 to 2: 1.
The method according to claim 1,
Wherein the composite lithium salt has a molar concentration of 0.1M to 0.7M.
An anode, a cathode, a separator interposed therebetween, and an electrolyte,
Wherein the electrolyte is the electrolyte according to any one of claims 1 to 3.
5. The method of claim 4,
Wherein the anode comprises a catalyst supported on a carbon support.
6. The method of claim 5,
Wherein the catalyst is ruthenium (Ru).

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